Pick the active voice of the following sentence:

This word cannot be uttered by a sane man.

In the passive sentence, this word receives the action, while a sane man does the action.

To change it into active voice, bring a sane man to the front and keep the meaning of cannot unchanged.

This word cannot be uttered by a sane man.

becomes

A sane man cannot utter this word.

Option 1 changes "cannot" to past tense.

Option 2 changes "cannot" to "will not".

Option 3 changes "this word" to "the word".

Therefore, Option 4 is correct.